According to Sir Isaac Newton’s theory of universal gravitation, gravity is an effect at a distance where one object feels the influence of another, regardless of distance. This became a central feature of classical Newtonian physics, which remained the accepted canon for over two hundred years. In the 20th century, Einstein began to reconceptualize gravity with his general theory of relativity, where gravity changes the curvature of local spacetime. Hence the principle of locality, which states that an object is directly affected by its surroundings and distant objects cannot communicate immediately.
However, the birth of quantum mechanics has prompted further conceptualization as physicists discovered that non-local phenomena not only exist, but are fundamental to reality as we know it. These include quantum entanglement, where the properties of one particle can be instantly transferred to another, regardless of distance. In a new study from the International School for Advanced Studies (SISSA) in Trieste, Italy, a team of researchers proposes that dark matter may interact with gravity in a non-local way.
The team was led by Francesco Benetti and Giovanni Gandolfi, two PhD students. Astrophysics and Cosmology Group students at SISSA. He was joined by researchers from the Institute for Fundamental Physics of the Universe (IFPU), the National Institute for Nuclear Physics (INFN), and the Institute of Radio Astronomy at the National Institute for Astrophysics (IRA-INAF). Her article “Dark Matter in Fractional Gravity. I. Astrophysical Tests on Galactic Scales” (the first in a series devoted to DM interactions) appeared in the Astronomical Journal.
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Image of KK 246 (ESO 461-036), a dwarf irregular galaxy located in the Local Cavity, acquired with Hubble’s Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS). Photo credit: NASA/ESA/Hubble/E. Shaya et al.
According to the most widely held theory in cosmology, dark matter is the mysterious mass that makes up about 85% of the matter in the universe. This matter does not interact with baryonic matter (aka normal or “visible” matter) through electromagnetism or nuclear forces, only through gravity (the weakest fundamental force). It was present just after the Big Bang, where it formed halos that caused all neutral hydrogen to accumulate in clumps and gave rise to the first stars in the universe, which merged to form the first galaxies.
In theory, DM is a fundamental part of nature and is responsible for the formation of cosmic structures ranging from galaxies to galaxy clusters. It is also responsible for the rotation curves of galaxies, which cause the stars in a galaxy’s disk to revolve around a common center. Its existence is also required for General Relativity, which has been endlessly verified by observation and experiment, to work on the largest scales. However, the nature of DM in terms of its composition (WIMPs or axions?) and its interaction with smaller galaxies remains a mystery.
According to the authors, their study proposes a new model of the non-local interaction between a galaxy’s DM and gravity that could provide a new perspective on the still-mysterious nature of this invisible mass. When Einstein put General Relativity in a nutshell, he said, “Matter tells spacetime how to curve, and curved spacetime tells matter how to move.” Benetti described to Universe Today via email his team’s theory: “In the innermost part of small galaxies, dark matter behaves like a non-local object that interacts with all other masses in the universe.”
This contrasts with the prevailing view that dark matter is “cold,” meaning it is composed of weakly interacting massive particles (WIMPS). These particles move slowly relative to the speed of light and interact weakly and locally with normal matter. As Benetti indicated:
“Although the most commonly used dark matter model (the so-called Cold Dark Matter, CDM) makes predictions that are well confirmed by experimental data at the cosmological scale, there are problems within galaxies, particularly in the cores of the smallest galaxies. Our model is able to overcome these problems by proposing a non-local interaction between dark matter within galaxies.”
The Hubble eXtreme Deep Field (XDF) combines Hubble observations made over the past decade of a small patch of sky in the constellation of Fornax. Image credit: NASA/ESA/UCSC/Leiden University/HUDF09 team
To model their nonlocality theory of DM, the team used fractions, a branch of mathematical analysis first developed in the 17th century. Fractions have been found to have applications in various areas of physics in recent years, but have never been tested in astrophysics. When describing DM in a limited system (small galaxies), this nonlocality shows up as a collective behavior of particles. Benetti and his colleagues applied their theory to the rotation curves of thousands of different types of galaxies, from small DM-dominated dwarf galaxies to large spiral galaxies.
Their results showed that their theory was better at predicting the rotational speed of galaxies (particularly smaller ones) than CDM in Newtonian gravity – which they confirmed by using Bayesian statistical analysis. Benetti said:
“In particular, the theory correctly predicts many scaling laws observed in galactic environments (the relationship of radial acceleration, core surface density vs. core radius, core radius vs. disk scale length) and that dark matter density should be suppressed at the center of dwarf galaxies compared to that , which was predicted from a cold dark matter model in Newtonian gravity. This is confirmed by observations and represents one of the major problems in cold dark matter models in Newtonian gravity.”
In particular, the theory proposed by Benetti and his colleagues could provide clues to the so-called “cusp core problem” (or “cuspy halo problem”). This refers to the discrepancy between the inferred DM density profiles of low-mass galaxies and the density profiles predicted by cosmological simulations. Several possible solutions to this problem have been proposed, including possible feedback mechanisms to alternative DM theories (including the possibility that it could be “warm”).
The nonlocality theory proposed by Benetti and his team offers a potentially revolutionary solution in the CDM framework and could have significant implications for cosmology. “Furthermore, if the mechanism by which dark matter evolves non-local behavior in these systems was the result of quantum nature, it would be an example of a quantum system on macroscopic, galactic scales, a very interesting phenomenon in its own right,” Benetti added . “In particular, other models of dark matter with a quantum nature are already known to the community, but none of them introduce non-local interaction via a fractional derivative.”
This research is part of a growing effort to unravel the nature of dark matter and dark energy, two of the greatest mysteries facing astronomers and cosmologists today. This effort will benefit significantly from next-generation telescopes such as ESA’s Euclid mission (launched today!) and the Nancy Grace Roman Space Telescope (RST), scheduled for launch in 2027.
Further reading: IN